Input/output processing is often the responsibility of an I/O subsystem of a communications environment. An I/O subsystem relieves the central processors of the communications environment of the task of communicating directly with the input/output devices of the environment and permits data processing to proceed concurrently with input/output processing.
The I/O subsystem employs various components, including I/O communications adapters and I/O communications subadapters. The I/O communications adapters are used in providing paths to the I/O devices, while the I/O communications subadapters are used to provide information concerning the I/O devices associated with the subadapters. There are many types of communications adapters and subadapters. In systems, such as the eServer zSeries and ESA/390 systems, offered by International Business Machines Corporation, Armonk, N.Y., an example of a communications adapter is a channel, and an example of a communications subadapter is a subchannel.
A subchannel provides the logical appearance of an I/O device to a program and includes the controls required by both the operating system and the I/O subsystem for sustaining a sequential stream of I/O operations directed against the associated device. In the current zSeries servers, a system footprint, referred to as a central processing complex (CPC), is limited to a maximum of 65,536 (64 k) subchannels per operating system image. As the zSeries processor core becomes more powerful over time and as the number of processors per CPC increases, operating systems are driving more and more workload in order to take advantage of the added processing capacity. As the workload of each operating system image increases, so does the need for I/O access. Thus, the 64 k maximum subchannel limit is a significant capacity constraint.
Based on the foregoing, a need exists for a facility that enables more than 64 k subchannels per operating system image to be used. In one example, the facility is to provide the additional capacity, while avoiding major programming impacts to existing operating systems. That is, the capability to allow more than 64 k subchannels is to be accomplished in a manner that is transparent to the existing operating system programs that do not take advantage of the increase in I/O connectivity.